The mechanical and chemical properties of surfaces are an important factor in almost all materials applications. Numerous techniques have been developed to enhance these properties for particular applications. These techniques range from widely used techniques such as galvanizing and heat treatment of alloys to specialty techniques such as ion implantation and laser glazing which are expensive and not scaleable to common industrial use. The use of surface treatments to improve properties such as surface hardness, wear resistance, corrosion resistance, and fatigue lifetime add significant value to a wide range of products in industries including automobile manufacture, aerospace, microelectronics, tool and die manufacture, power generation, and the production of steel, aluminum, ceramics, and plastics.
Thermal treatment to alter the surface properties of materials has been a standard industrial process since the early smiths developed techniques for pack carburization of cast iron in their forges to produce a material more suited to the fabrication of durable swords. Such treatments, principally metallurgical in nature, have thus formed a part of manufacturing technology for more than a thousand years.
Two primary driving forces are available through the use of thermal treatments. The first involves the use of high temperature to overcome kinetic barriers which keep something from happening. The second is based on the rapid quenching of hot material to preserve, in some degree, the microstructure of the hot material or that of a metastable structure encountered in the evolution toward the quenched material.
The metallurgy of sword blades, although not understood at the time and developed through empirical research, represents the pinnacle of materials engineering during the Dark Ages. It also serves to demonstrate various surface treatment modalities which are still of primary importance.
In the Dark Ages, iron was generally produced as wrought iron, i.e., as a matrix of nearly pure iron with a relatively high density of slag inclusions. (Slag is the combination of impurities and the flux used in the reduction of iron ore.) Wrought iron is reasonably tough, but is very soft, and hence not well suited to the production of weapons. It is possible to harden the material by cold-working in the course of shaping, thus producing a work-hardened material, but this material is brittle as well as hard. Thus, a method that hardened only the surface layers would produce a superior weapon, one which would both hold an edge and take an impact without fracture.
The process most often used to this end was pack carburization. A sword was forged (hammered) into shape, producing a hard but brittle implement. The sword was then packed in a mixture of carbon (charcoal or coke) and other organic materials. The pack was heated to the carburizing temperature (about 950.degree. C.), whereupon carbon was transferred to the steel by decomposition of carbon monoxide at the iron surface. The carbon diffused into the iron, forming a crude form of hypereuctectoid iron surrounding a wrought iron core. This is a thermodynamically favorable process which requires heating to yield significant diffusion in a reasonable amount of time.
The sword was then heated to above the transformation temperature (723.degree. C.). As a result, the iron core was left unchanged, whereas the steel tegument transformed into a mixed phase of austenite and cementite. (Austenite is the .gamma.-phase of iron with dissolved some carbon, while cementite is Fe.sub.3 C, a hard brittle substance.) The sword was then quenched abruptly to prevent the austenite-cementite mixed phase of the tegument from transforming back into the original material. (This step uses the second of the primary thermal treatments, using rapid quenching to preserve a high-temperature structure.) This quenching was usually performed by plunging the hot sword into oil, but some texts insisted that the finest swords resulted from quenching by thrusting the hot sword through the body of a young boy or a virgin maiden. The popularity of these latter practices is not recorded.
The structure resulting from the above steps was a strong, tough iron core surrounded by a thin (&lt;1 mm) layer of fine-grain high-carbon steel having high hardness but low toughness. The high carbon-steel tegument of the sword can now be sharpened to a fine edge, which will last through much use, while the soft iron core will allow deformation on impact, thus avoiding brittle fracture of the sword blade. The relative quality of local surface treatments of wrought iron was an important factor in determining the outcome of many early political disagreements.
Surface treatment of a wide range of materials is still extremely important in modern manufacturing. Considering only steel for a moment, carburizing is still widely used to produce a hard surface on steels, but has been joined by a wide range of related techniques including nitriding (including ion nitriding), carbonitriding, cyaniding, and liquid carburizing. Thermal treatment alone is also used to harden the surfaces of medium and high-carbon steels, where there is already enough carbon to form the austenite-cementite mixed phase using heating alone. However, in these cases only the surface must be heated, leading to the development of numerous techniques for heating only the surface of a body, including pulsed inductive heating, where heat is generated by induction of eddy currents, which are confined primarily to the near-surface region in a conducting body, direct heating by a flame followed by quenching quickly enough that the inner regions of the body are not heated past the transformation temperature, and laser hardening, in which a very thin layer of the surface is heated quickly enough that quenching is accomplished by simple thermal conduction into the body of the part being hardened. Laser hardening is an early example of the type of process of interest in the present application, i.e., a rapid heat-rapid quench process. Finally, a work hardened surface can also be formed by heating a thin surface layer sufficiently that it ablates from the bulk of the material, creating shock waves which create dislocations in the near-surface regions.
Other purposes than hardening can be served by surface treatment of materials. For example, in steels, the chromizing process (diffusing chromium from an external source) increases the corrosion resistance of the material by turning the surface region into a form of stainless steel which may or may not be harder than the initial material, but will resist the action of oxidants and other common sources of corrosion. However, many other materials, including metals, alloys, semiconductors, ceramics, and other nonmetals are subject to corrosive effects. These include galvanic corrosion, in which a current is generated between two dissimilar metals in electrical contact, resulting in a soluble species being generated from one of the metals. Another form of corrosion is pitting, in which a passivation (or protecting) layer is breached in a small area (a pit). The material underneath, which is usually protected by the passivation layer, is then exposed to the corrosive environment, and dissolves inward from the position of the pit. (Note that a passivation layer may be intrinsic, as in the oxide layer on aluminium, or extrinsic, as in the surface layer of zinc in galvanized steel.) Corrosion can also be stimulated by tensile stress, biological organisms, and atmospheric contamination. A side effect of corrosion mechanisms can be hydrogen embrittlement, in which the hydrogen generated by a corrosive process enters into a metal or alloy, reducing the ductility of the material, which is then weakened, allowing surface flaws to grow under a stress, thus increasing the susceptibility of the material to further corrosion and eventual failure under stress. Many methods exist to reduce the pernicious effects of corrosion, but it is estimated that some 4% of the gross national product is still lost to corrosive effects.
One type of surface treatment to improve corrosion resistance which is consistent with the class of treatments under discussion here, i.e., rapid heat-rapid quench techniques, is the formation of surface alloy layers by forming a layer of a second material on the object to be protected, and then melting the surface layers to form a homogeneous liquid material. If the quenching is then rapid enough, a solid solution results even if an equilibrium alloy cannot be formed at low temperature. The solid solution may be amorphous, nanocrystalline, microcrystalline, or actual precipitates may form, depending on the rate of quenching. A small amount of precipitation will still allow corrosion resistance to be improved, and may increase the hardness of the treated surface layer.
Surface treatments are also important in the control of wear, which constitutes the primary reason why the artifacts of society become useless and have to be replaced. Wear is simply the removal of material from a solid surface as a result of sliding action. Wear is occasionally a useful process (e.g., writing with pencil and paper), but more often is deleterious to both the structure and the operation of mechanisms. There are four primary types of wear, adhesive, abrasive, corrosive, and fatigue wear. Adhesive wear arises from the formation, during sliding, of regions (called junctions) of adhesive bonding on a microscopic scale. If the junctions do not break along their original interfaces upon further sliding, then a chunk from one of the surfaces will have been transferred to the other surface. Such particles constitute wear in their formation, and may also add to abrasive wear. Abrasive wear is produced by a hard object being dragged along a softer one, thereby digging out a groove. The abrasive agent may be one of the surfaces, particles removed from the surfaces by other wear mechanisms, or external particles, such as sand in a bearing. Corrosive wear occurs when sliding action takes place in a corrosive environment, the pieces are nominally protected by a passivating layer, and the sliding action continuously removes the passivating layer, thus exposing fresh surfaces to the action of the corrosive. Fatigue wear occurs as cracks form and grow as the result of fatigue, especially in rolling systems. A crack forms below the surface, and grows to intersect the surface, thereby lifting a large particle out of the surface.
The various forms of wear are often synergistic, resulting in a form of degradation which is nearly universal in any mechanism or device having moving parts. Adhesive wear is the most fundamental, existing in any sliding or rotating contact in which two surfaces touch. The primary line of defense against wear is the use of lubricants, which act to prevent contact of surfaces in relative motion, thus reducing wear by as much as a million times the dry value. However, unless the relative velocities of the surfaces is high enough that the surfaces `surf` on a continuous film of lubricant, there will still be contact and adhesive wear will occur.
The condition of the sliding surfaces helps to determine the rate of wear. The friction between a pair of sliding surfaces having large surface roughness will be approximately the same as that between similar surfaces having smoother surfaces. The friction in the first case is the work done in tearing apart a few large junctions, whereas in the second case the friction results from tearing apart many small junctions, but the total surface area of the junctions (and hence the work required to tear them asunder) is determined primarily by the amount of deformation of the surfaces caused by the force normal to the surfaces, which is the same in both cases.
From the point of view of the wear occurring during sliding, however, the difference in surface roughness makes several important contributions. Take .lambda. as a length characterizing the surface roughness. (In some cases the appropriate length will be the grain size rather than the surface roughness, but the arguments below still hold.) It is clear that .lambda. will also characterize the size of the particles torn off by adhesive wear. That is, if .lambda. is 10 times larger, the size of the detached particles will also be 10 times larger. Smaller particles will contribute less to abrasive wear mechanisms.
The difference in size of the detached particles resulting from differing degrees of surface roughness yields another advantage. A simple scaling argument will illustrate this. Assume .lambda..sub.R for the rough surface is 10 times larger than .lambda..sub.S for the smoother surface. Under equal external loading, the areas of contact will be the same. As each junction now has an area on the order of .lambda..sup.2 there must be 100 times as many junctions on the smooth surface as on the rough surface. Further assume, as above, that the characteristic size of the detached particles is .about..lambda.. The volume of material detached by adhesive wear is then .about.N.lambda..sup.3, where N.about..lambda..sup.-2 is the number of particles formed by adhesive wear. The total volume of material removed by adhesive wear in a given sliding process is thus proportional to the size of the surface roughness. In summary, smoother surfaces, despite producing the same friction, result in less direct adhesive wear because of the square-cube scaling law above, and also reduce the amount of self-abrasive wear by reducing the size of the abrasive particles generated during adhesive wear. Such surfaces can be fabricated by melting the surface layers of the body and allowing the heat energy to dissipate into the body, thus obtaining rapid quenching of the liquid.
Fatigue wear can also be affected by surface morphology, but not primarily by the length scale of the surface roughness. More important here is the presence of abrupt structures, such as cracks, ledges, overhangs, etc., which offer sites for stress concentration, and the earlier material failure accompanying such concentration. Such stress concentration does not depend on the length scale of the defect, but rather on its shape. This type of wear will be reduced if the surface can be treated to have more gradual changes in surface morphology. Again, surface morphology can be altered to provide less opportunity for stress concentration by rapidly melting the surface layers and quenching the heat energy into the body of the material in question. Such smoother surfaces will also serve to limit various mechanisms for corrosion.
The potential applications for rapidly heating (and perhaps melting) a thin surface layer which is then self-quenched via thermal conduction into the body of an object are very broad. Beyond those described in detail above, one may alter the surface layers of a material. This may be done in a number of ways, but the main route toward such alterations is the ability of rapid quenching to produce non-equilibrium structures, such as amorphous or nanocrystalline surface layers. Metastable surface alloys can be produced by rapid melting and quenching. This requires a material system in which a thin layer of material A is formed on a substrate B. The phase diagram of these materials is such that they are immiscible when solid, but form a single-phase liquid when molten. (Heating above the melting point may be required.) If this material is then rapidly quenched, an amorphous alloy composed of the two components will result. If the quenching process is somewhat slower, nanoscale precipitates will form. The size of these precipitates depends on the cooling rate. Note that these materials need not be metals. A coating of gold on a germanium substrate melted and rapidly quenched will form such an amorphous alloy. Compounds including members of the metalloids are well known as helpful in formation of amorphous materials.
When the pulse energy is much greater that that required to melt the heated surface layer, the surface layer will ablate. This can have three desirable effects. One is to serve as a source of pure material for an associated deposition process analogous to sputtering, but providing greatly enhanced purity and smoothness of the deposited material. It is also possible to obtain unique surface structures, which have, for example, altered electron emission characteristics, by ablating a surface layer from a substrate. Some of the ablated material will redeposit on the substrate, forming the aforementioned unique structures. Third, the shock wave created in the substrate by such ablation produces work-hardening effects far into the material (perhaps several hundred microns) through formation of dislocation structures below the heated surface layer.
Another application of a rapid heating-rapid quenching cycle is to clean a surface without altering the properties of the surface in any manner. This would be possible when the contaminant will desorb from the surface at a temperature lower than the melting point of the surface. Properly done, only a very thin (&lt;&lt;.mu.m) layer of the surface would have to be heated to remove contaminants compatible with this method without the use of solvents or other chemicals. Such a process could replace many cleaning steps presently required in machining and semiconductor manufacture, to name only two possibilities. This is an important consideration in these days of heightened ecological awareness and regulation.
The surfaces of porous and/or highly defective materials such as ceramics can be smoothed and rendered resistant to crack nucleation by forming a surface layer of glass using the same type of rapid heat-rapid anneal treatment. A similar smoothing of surfaces was described in the discussion of wear above. Further applications include `polishing` of machined parts. Precision machined parts will commonly retain machining marks on the order of 10 .mu.m in size. Surface melting can allow the surface tension of the material to induce material reflow, smoothing the surface. Such techniques could also find application in the final polishing of diamond-turned optics, thereby totally avoiding conventional optical surface generation techniques. Finally, smooth surfaces offer fewer flaws to initiate corrosive processes. This class of surface finishing techniques will thus reduce the initial rate of corrosion significantly, beyond any changes in surface chemistry which may also be accomplished.
Having established that rapid-heat rapid-quench processes are potentially of great industrial use, one must naturally ask why they are not presently being applied in standard industrial practice. There are numerous reasons why previous laboratory-scale attempts to apply such processes failed to be accepted in the market.
Consider the conditions required to melt a thin layer (1.about.1 .mu.m) of a steel surface. How much energy is required to melt the surface assuming that no thermal conduction into the bulk of the steel occurs? The melting point T.sub.m of steel is about 1530.degree. C., the density .rho.is .about.7000 kg/m.sup.3, and the specific heat c is .about.3Nk where N is the number of atoms per kg (.about.1.07.times.10.sup.25), and Boltzmann's constant k is 1.38.times.10.sup.-23 J/.degree.K. The energy per m.sup.2 in a 1 .mu.m thick layer of molten steel is roughly .rho.cT.sub.m .DELTA.l, or some 5600 joules per square meter. As the present interest is in industrial-scale processes, treatment of a square meter of material at a time is not unreasonable, at least for discussion.
Now the time required for quenching of this energy into the bulk of the steel must be estimated. The rate of power flow out of the molten layer is roughly kT.sub.m /2l joules per square meter per second. Combining this with the earlier result, and assuming that k is not a function of temperature, ##EQU1## where .tau. is the characteristic time for the heat energy to leave the molten layer through conduction into the bulk of the steel. K is .about.100 watts per meter per .degree.K, so the characteristic time for this situation is roughly 30 nanoseconds. Note that this estimate gives a cooling rate of .about.5.times.10.sup.10 .degree.K per second, a remarkably large value compared to .about.10.sup.6 .degree.K-sec.sup.-1 for techniques such as splat quenching or planar flow casting techniques. Thicker surface layers will require longer cooling periods; for example a 10 .mu.m melted layer on a steel body will cool at .about.5.times.10.sup.8 .degree.K-sec.sup.-1, a value still associated with non-equilibrium effects.
In the primary mode of operation, the energy of the thin molten layer must be deposited in a period of time shorter than .tau. so that the deposited energy efficiently heats only the desired surface layer, and not the underlying material. Accordingly, the deposited power P must be greater than the energy deposited divided by the characteristic time period, or EQU P&gt;0.18 terawatts per square meter.
A secondary mode of operation is also available, in which the beam energy is deposited in a thin surface layer on a time scale much longer than the characteristic thermal diffusion time for the surface layer being heated. This mode of operation is analogous to flame annealing. Such a mode is useful for annealing the surface layer, to induce grain growth or to produce a thin nanocrystailine surface layer on an amorphous material. In this mode substantial temperature increases will be experienced by much more of the material than the surface layer being directly heated, an effect which must be accounted for when carrying out the ion beam surface treatment. (A worker skilled in the art can use the equations given in this specification to predict thermal profiles (i.e., temperature increase vs. depth vs. time) for a given set of production conditions.)
Making this class of techniques even marginally practical (larger molten thickness would be desirable in most cases) requires a source unit that can deliver .about.10.sup.4 joules into the surface of a body in a 30-3000 nanosecond pulse. Further, unless rapid cycling (&gt;&gt;1 Hz) of the source is possible, the amount of material that can be treated per source unit is too small to have an impact on any but specialty items. High process efficiency is also required, as otherwise removing the waste heat from the source unit will become a difficult task, as will providing the total power required.
Consider a more definite case. The source unit is to be a pulsed laser. (The difficulties surrounding the problem of depth and consistency of power absorption will be ignored for a moment.) To get 10.sup.4 joules output, our 1% efficient laser will require 10.sup.6 joules input. To provide 10 pulses per second, a minimal speed for practical applications of this technology, the source unit must receive some 10 megawatts continuous input and have a cooling system capable of removing and disposing of nearly that much power continuously. The cost in wasted electricity alone is about $10 million dollars per year of operation. The low power efficiency of laser systems which provide short enough pulses of sufficient energy to treat large areas of a surface is clearly a problem.
Lasers present other problems when considered for this class of applications. A pulsed laser system with the required level of power has been developed for antiballistic missile systems, but the physical size and capital cost of each system is enormous. In addition, the lifetime of certain critical components is quite short (&lt;10.sup.3 pulses), requiring enormous downtime for maintenance in an industrial situation. Further, the depth of power deposition is limited to an optical skin depth. As this is much less than a micron for any suitable laser system now available acting on metals, one of two situations will develop. The total energy will be delivered suddenly to one skin depth of surface, which will then vaporize and ablate from the surface. Alternately, the energy can be slowly fed into the surface through the bottleneck presented by the requirement that the outer few nanometers of the surface not vaporize, thus requiring longer pulses with lower power. This option results in long heating periods, and substantial heating of the material underlying the desired surface heating region. Such a situation is non-optimal. Finally, in order to use a laser as a source unit for this class of manufacturing applications, the surface condition of the material presented must be carefully controlled so that the power absorption is uniform throughout the material being treated. Such control in a general industrial manufacturing environment would prove difficult.
Another possible source is an ion beam generator. Such generators are able to deposit their energy with reasonable uniformity down to depths of many microns, depending on the energy and species of the ions used, offering some promise for application to the present class of manufacturing processes.
It is important to note that the ion beam generator is not being used for ion implantation in the usual sense. There is a great deal of information on alteration of surface and near-surface regions by ion implantation, in which a rapid thermal effect is not the operative driver, but rather the gross changes in chemistry caused by implantation of the ions or the localized lattice damage resulting from slowing of individual ions. The point is that in conventional ion implantation the rate of implantation (i.e., the beam current per unit area) is of little importance, as long as the ions eventually are implanted. In the present applications, the thermal effects caused by the extremely high current of ions impacting the surface are primarily responsible for the favorable surface modifications. In most cases the total dose of ions will be small enough to leave the surface composition essentially undisturbed. This point will be discussed in a more quantitative manner below.
For the moment the problem of making a suitable ion beam generator will be ignored, and attention placed on the characteristics such a generator must have to function in the modalities described above. Two problems present themselves. First, for given species of ion and target, how much beam energy is required to penetrate a given distance into the target, thus heating the target surface to that depth? Second, what total dosage is required to melt the affected area? The answers to these questions will determine the characteristics required by an ion beam generator useful for surface treatment in a manufacturing environment.
The rate at which energy is lost to electronic collisions (the primary mode of energy loss in the relevant regime) by an ion of mass M.sub.1 and atomic number Z.sub.1 while traversing an amorphous (or polycrystalline) target consisting of atoms of mass M.sub.2 and atomic number Z.sub.2 can be expressed in dimensionless units for length (.rho.) and energy (.epsilon.) within the LSS theory (Ion Implantation in Semiconductors, by J. W. Mayer et al., Academic Press, 1970, pgs 21-26) as ##EQU2## The dimensionless parameters are given by EQU .rho.=4100dM.sub.1 /[(M.sub.1 +M.sub.2).sup.2 (Z.sub.1.sup.2/3 +Z.sub.2.sup.2/3)]R(.mu.m), EQU .epsilon.=9500M.sub.2 /[Z.sub.1 Z.sub.2 (M.sub.1 +M.sub.2)]E(MeV),
and EQU k=0.0793Z.sub.1.sup.2/3 Z.sub.2.sup.1/2 (M.sub.1 +M.sub.2).sup.3/2 /[(Z.sub.1.sup.2/3 +Z.sub.2.sup.2/3).sup.3/4 M.sub.1.sup.3/2 M.sub.2.sup.1/2 ].
In the above equations, R is distance in microns, E is the ion energy in megaelectron volts, and d is the density of the target material in grams per cubic centimeter. The energy loss equation can be solved for energy remaining after a given distance of travel in the target by substituting .rho.=.eta..sup.2, and integrating to find EQU .epsilon.(.rho.)=k.sup.2 .rho..sup.2 /4-k.rho..epsilon..sub.o +.epsilon..sub.o,
where .epsilon..sub.o is the initial dimensionless energy of the ion. Given this equation, the range of an ion in the target material is found by setting .epsilon.(.rho.)=0, and solving the resulting binomial equation for the total range .rho..sub.t to give EQU .rho..sub.t =2.epsilon..sub.o.sup.1/2 /k.
Both theory and experiment agree that the energy of the ions is distributed relatively uniformly throughout a volume starting at the surface and proceeding .about..rho..sub.t inward.
To give a feel for the above equations, consider a specific example. Carbon ions (Z=6, M=12) having an energy of 1 MeV are incident on an iron (Z=26, M=56) surface. The dimensionless energy .epsilon. is equal to 50 E(MeV), the dimensionless range .rho. is equal to 6.2 R(.mu.m), and k=0.37. (All numerical values are approximate.) By the above range equation, .rho..sub.t =2.times.50.sup.1/2 /0.37=38.2. Solving for the actual distance R.sub.t (.mu.m)=.rho./6.2=38.2/6.2=6.2 .mu.m range. The energy distance relationship calculated here is by no means universal, but serves to illustrate that when thermal heating of a surface layer having a thickness of several microns is desired, the ion energy required is likely to be on the order of 1 MeV.
The question of the total dosage required to melt a surface layer can now be illustrated. Continuing with the above example, the earlier estimate that 5600 J/m.sup.2 is required to melt a 1 .mu.m layer of steel shows that some 35000 J/m.sup.2 is required to be deposited to melt the 6.2 .mu.m surface layer heated by a 1 MeV carbon beam. This amount of energy is also equal to 2.2.times.10.sup.17 MeV. The process of melting the 6.2 .mu.m surface layer thus requires an addition of carbon ions amounting to about 2% of a monolayer. The affected region is some 10.sup.4 S of monolayers thick, so in this example the contamination of the surface layers by carbon is on the order of 1 part in 10.sup.6, an amount negligible to the chemistry of most surface modification processes. This demonstrates that the effect of high energy pulsed ion beams is due almost totally to thermal heating of the surface layers, a process made very different from ion implantation by the time scales involved.
Finally, the beam current required can be estimated. A pulse of 1 MeV carbon ions must consist of 2.2.times.10.sup.17 ions if a square meter is to be treated in a single pulse. This amount of ions must be transmitted in no more than a few hundred nanoseconds. (The timescale is longer because of the increased thickness of the melted zone.) The resulting rate is about 10.sup.25 ions/sec, representing a current of about 1.6 megaamperes. The pulse must therefore carry a power of some 1.6 terawatts per square meter of surface treated. The size of this number explains why so few experimental studies of surface modification using the thermal effects of ion beam treatment have been made.
Given that the use of ion beam generators for surface modification can be carried out as described above, why is there currently so little penetration of commercial markets? The use of ion beams for thermally altering the near surface characteristics of a material has been fraught with substantial problems. Most notable of the limitations with existing ion beam technologies have been: 1) high costs per area treated; 2) the inability to generate a large number of pulses without the costly replacement of ion beam generator components; 3) low repetition rates; 4) low average power; and 5) the inability to reliably produce a uniform ion beam of a single selectable ion species.
Typical ion beam generators use dielectric surface arcing on an anode as a source of ions and thereafter magnetically or geometrically direct and focus the generated ion beam onto the material of interest. This surface arcing (also called "flashover") destroys the anode surface in less than 100 pulses, and produces a mixed species of ions that cannot be adjusted. Other difficulties arising from flashover include: production of large quantities of neutral gas that makes high repetition rate difficult, generation of debris which can contaminate surfaces being treated, and non-uniformity and irreproducibility of the beam in some cases due to the localized and difficult to control nature of flashover.
State-of-the-art ion beam generators are typically "one shot" devices, i.e., they operate at low repetition rates (&lt;&lt;1 Hz). Existing ion beam generators cannot be operated at high repetition rates (&gt;&gt;1 Hz) for a number of reasons. First, existing pulsed power supplies are not able to generate electrical pulses at high repetition rates having the voltage, pulse width (i.e., nominal temporal duration), and power required to generate the ion beams needed (i.e., consistent with the discussion above) for the various beneficial applications described herein. This limitation renders commercial exploitation impractical. Second, the design of existing ion beam generators does not allow repetitive operation for an extended number of operating cycles (&gt;&gt;10.sup.3) without replacement of major components. This limitation would require a maintenance time--manufacturing time ratio incompatible with routine manufacturing operations. Fourth, existing ion beam generators generally operate with electrical efficiencies&lt;5%, thus presenting major challenges to the pulsed power supply and the cooling system of the generator. These limitations and others have made it impossible to routinely utilize the ion beam technology described above for surface treating materials.
The present invention generates high energy, repetitive ion beams which overcome the limitations of existing ion beam generators and provides a cost-effective processing technology for thermally altering the near surface characteristics of materials.